1. Field of the Invention
The present invention relates to a metallic closed vessel, or a so-called canister, in which a radioactive substance that involves heat release is sealed, a seal-welding method for the closed vessel, and an exhaust system used for the seal-welding method.
2. Description of the Related Art
Highly radioactive substances represented by spent fuels from nuclear reactors are destructured and reprocessed in order to recover plutonium or some other useful substances that can be used again as fuels. These spent fuels are contained in closed places before they are reprocessed. Known containing methods for these highly radioactive substances include a wet method that uses storage pools and the like and a dry method that uses casks and the like.
The dry method is a containing method in which air is used in place of water for natural cooling. Since the running costs of the dry method are lower than those of the wet method, the dry method has started to attract attention and be developed. Known casks that are applicable to the dry method include metallic casks and concrete casks based on a concrete structure for shielding the spent fuel. Each of these casks is provided with a tubular vessel body that is closed at both ends, top and bottom. The spent fuel is sealed in a tubular metallic closed vessel or a so-called canister, moreover, the canister is put into the vessel body of the cask. Thus, radioactive substance can be contained in a shielded state.
Usually, the canister comprises a tubular vessel body closed at the bottom and a lid that closes a top opening of the vessel body. A basket is located in the vessel body, and a plurality of spent fuel assemblies are sealed in the vessel body in a manner such that they are supported by the basket. Normally, the spent fuel assemblies are sealed into the canister in the following processes.
First, the open-topped vessel body of the canister is immersed in cooling water and filled with the water. In this state, the basket and the spent fuel assemblies are contained in the vessel body. Thus, the spent fuel assemblies are temporarily shielded with the cooling water to prevent leakage of radiation.
Subsequently, a primary lid is dropped onto the top opening of the vessel body to close it, and a suitable quantity of water is discharged. Thereafter, the primary lid is welded to the vessel body to seal the top opening of the vessel body. After the water is completely discharged from the vessel body through a drainage hole in the primary lid, the drainage hole is sealed. Further, a secondary lid is lapped onto the primary lid and welded to the vessel body. Thus, the resulting canister has the spent fuel assemblies well sealed therein.
In the sealing process for the canister described above, the vessel body is filled with the cooling water as the primary lid is welded to it, in order to intercept radiation from the spent fuel assemblies. However, the welding operation takes so much time that the cooling water in the vessel body is heated by the spent fuel assemblies and evaporated gradually. The resulting steam fills the vessel body and flows out of it through the gap between the inner surface of the vessel body and the primary lid.
Normally, a welding operation is performed in a manner such that molten deposited metal naturally drops by the gravity, thereby forming penetration beads. As this is done, however, steam gets into the gap between the inner surface of the vessel body and the primary lid, as a welding portion, so that weld defects such as voids are inevitably formed in the welding portion. These weld defects lower the strength of the welding portion, and a radioactive substance leaks from the defective portions. Thus, it is hard to maintain the integrity or radioactive substance sealing performance of the canister.